1. Field of the Invention
This invention relates to a subsea connection apparatus for connecting a surface blowout preventer stack and riser to a subsea wellhead. This unique subsea connection apparatus uses a single cavity blowout preventer with a set of shearing blind rams disposed therein. Hydraulically actuated wellhead connectors are secured to the top and bottom of the single cavity blowout preventer. The wellhead connectors are oriented to allow connection to a subsea wellhead disposed below the subsea connection apparatus and a wellhead hub profile on the lower end of a riser disposed above the apparatus. The riser extends upwardly to connect to a surface blowout preventer stack on the drilling rig above.
The idea of locating a Blowout Preventer (BOP) stack on the ocean surface to provide well control while drilling for offshore oil is not new. When the first land rig was mounted on a barge decades ago, these systems were common. Later, jack-up rigs were outfitted with such systems. Jack-up rig evolution allowed their water depth capability to be expanded to 650 ft. Then, semi-submersible rigs and drillships were developed and the blowout preventers were moved to the sea floor allowing a relatively low-pressure (and thus, less expensive) riser to transport the drilling mud returns back to the mud processing equipment located in the rig by way of the riser annulus. This seabed BOP configuration facilitated the original water depth expansion to 1500 ft. with second generation rigs, and later to 3,000 ft. with third generation rigs. As time passed, the water depth capability had been extended to 10,000 ft. as larger and much more expensive fourth and fifth generation rigs gradually came into service in the 1990s.
In an effort to allow the more economical second and third generation rigs to drill in water depths in excess of 3,000 ft. the surface stack application has been resurrected. Unlike the systems used on jack-up rigs, these latest applications use casing pipe as the riser from the seabed to the surface. This provided several advantages over using subsea stacks. First, the casing could be run much faster than a subsea riser, reducing trip time. Second, the casing pipe used as riser for one well would be cemented into the seabed on the next well, negating the need for fatigue analysis on the riser pipe. In addition to this time and analysis savings, all this could be accomplished with a rig day-rate savings of $50,000/day or more.
However, there was a serious drawback to this application. With the riser cemented into the seabed and the BOP stack latched atop it at the surface, the consequences of riser failure become much more serious than with conventional low pressure riser/subsea stack applications. There is any number of situations that could cause riser failure. In all of these cases, the wellbore would be open to the sea, which is a situation to be avoided because, at best, losing the riser's mud column weight could lead to loss of well control, and at worst, the wellbore formation fluids and pressures would be vented to the sea. These results could easily be an environmental disaster, as well as posing the possibility of injury to rig personnel and rig equipment damage.
There is therefor a need for a simple, cost effective and expendable apparatus that allows the use of surface blowout preventers in combination with a low cost riser to be used in subsea drilling applications. Such a system should allow the use of existing subsea drilling equipment and technology and require minimal modifications to the rig.
2. Description of Related Art
A subsea drilling riser disconnect system and the method of its use are disclosed in Patent Cooperation Treaty International Publication Number WO 02/088516 A1 and invented by Peter E. Azancot.